Acoustical covers for office machines



May 26, 1970 w. BUEHNER T A 3,

- ACOUSTICAL COVERS FOR OFFICE MACHINES Filed June 2, 1969 3Sheets-Sheet 1 FIGJ INVENTORS. WILLIAM L. BUEHNER DONALD L. GREERRICHARD W. M CORNACK PAUL L. O'BRIAN HENRY W. SIMPSON QAM ATTORNEY.

May 26, 1970 w'. L. BUEHNER ETA!- 3,513,938

ACOUSTI CAL COVERS FOR OFFICE MACHINES Filed June 2, 1969 3 Sheets-Sheet2 May 26, 1970 w. L. BUEHNER L 3,513,938

' ACOUSTICAL COVERS FOR OFFICE MACHINES Filed June 2, 1969 3Sheets-Sheet 5 FIG. 9

OCTAVE PASS smos- CYCLES PER SECOND 70 45 9o 1ao 555 -no -1400- g995s0o: 120o FREQUENCY-CYCLES PER SECOND United States Patent Office3,513,938 Patented May 26, 1970 US. Cl. 18133 9 Claims ABSTRACT OF THEDISCLOSURE A low profile acoustical cover for an office machine adaptedto fit over the opening in the machine housing which accommodates paperfeeding and handling. The acoustical cover incorporates a plastic hoodhaving angled walls enabling operator observation of the printing on thepaper with a minimum of glare from the ambience of the machine and anacoustic device which is placed in proximity to paper entrance and exitslots in the plastic hood to prevent the emission of undesirable audiblesounds therethrough. The plastic hood pivots to enable ready access tothe machine mechanism and to the paper being operated on and the entireacoustical cover is readily removable from the machine.

CROSS REFERENCE TO RELATED APPLICATION The following application isassigned to the same assignee as the present application.

US. patent application Ser. No. 15,231, filed Dec. 30, 1968, entitledTypewriter or the Like, Eliot F. Noyes, inventor.

OTHER REFERENCES The following additional reference is of interest:

IBM Customer Engineering Instruction Manual for the IBM Selectric 1Composer, Form Number 241-53404), dated October, 1966. 1

Brief background of the invention Field This invention relates to anacoustical cover for an office machine, and more particularly, to animproved acoustical cover for the platen area of a typewriter.

Description of the prior art Typewriter machines or the like whichincorporate paper feeding platens which are strikable by type elementsgenerally emit a substanital amount of noise into their environment.This noise can be excessive, especially when the machine is operatedfrom a secondary media, such as magnetic or paper tape or from atransmission line at its maximum output rate for long periods of time inthe environment of a business oflice. Much of the noise which is emittedfrom the machine, is emitted through the opening in the machine housingabove the platen which facilitates paper handling and there have beenmany attempts made in the past to absorb this noise. The prior artattempts may be classified into two general categories: (1) largecabinets which completely surround the typewriter and (2) smallerenclosures adapted to fit over the opening in the typewriter housingabove the platen. The large cabinet devices are usually constructed witha transparent window through which the operator can observe the printingoperation. The paper is generally maintained completely within the eni(Trademark) closure, although several such prior art devices includeslots through which the paper is fed. In order to prevent sound fromescaping through the paper slots, rubber rollers or gasket type deviceshave been utilized to provide a relatively tight seal with the paper.While these cabinet devices have been successful in appreciably reducingthe sound emanating from a typewriter or the like device, none of themhave met with any degree of commercial success because of theirbulkiness and because of the paper handling problems which they create.That is, when such a device encloses a typewriter, it is necessary forthe operator to open doors to reach into the device in order to alignpaper, make corrections and in some instances to vieW the printed lines.Additionally, these devices are generally not designed for optimumviewing of the line being printed and those preceding it. In fact, inmany instances, these devices incorporate lights to enable the operatorto see the print line and also contain mirrors for reflecting the printline to a position where it can be viewed by the operator.

As described above, a second class of acoustic hoods exist in the artwhich cover only the platen opening in the machine housing. Some ofthese devices form only a partial covering for the housing opening tothereby facilitate the feeding of paper and are virtually ineffective toreduce emitted sound. Instead, they direct the emitted sound away fromthe operator without affecting the sound level within the ofiice orenvironment in which the machine is located. Other such devices form acomplete covering for the platen opening thereby appreciably reducingthe emitted sound, but they create paper handling problems. That is, itis necessary to either maintain the paper completely within the coverthereby creating operator visibility problems or to feed the paperthrough an opening having a seal adapted to coact with the paper toprevent the emission of sound. When such a seal is utilized, it forms anobstruction which makes it difiicult for the operator to properly alignthe paper with respect to the platen. Additionally, such prior artcovers usually make it quite difficult for the operator to access theprint line to make corrections thereon without necessitating the removalof the cover. Further, many such prior art devices incorporate mirrorsor the like to provide the operator with a view of the line beingprinted thereby creating operator non-familarity problems with theequipment.

Summarizing, the prior art teaches two basic approaches to reduce thenoise emitted through the platen opening of a typewriter or the likemachines: enclosure of the entire operating mechanism within a largehood with the resultant loss of ease of paper handling ability andviewing ability and enclosure of the platen opening with a resultinglack of noise reduction or a resulting lack of visibility and ease ofpaper handling.

SUMMARY In order to overcome the above noted shortcomings of the priorart and to provide an acoustical cover for office machines which greatlyreduces the noise emitted therefrom without appreciably disturbing thevisibility of the printed lines and without appreciably disturbing thepaper handling manipulations which the operator is accustomed to, thepresent invention provides an acoustical cover adapted to fit over theplaten opening and having angled walls enabling ready operatorobservation of that portion of the paper which has been printed upon,and an acoustical device mounted in close proximity to the paperentrance and exit slots of the cover for eliminating the audible soundwhich would otherwise escape through the slots into the ambience of themachine. The walls of the acoustical cover are so angled to preventglare or unwanted reflections at the normal operator viewing position aswell as to allow pivotal motion of a portion of the acoustical cover toan open or inoperative position to enable the operator to rapidly accessthe platen area for corrections, and for type element and ribbonchanging operations. The acoustic device forms an integral part of theacoustical cover and is mounted directly over the platen area and formsunobstructed slots with the angled walls of the acoustical cover toenable paper to be readily fed into and out of the platen area.Additionally, the proximity of the acoustic device to the paper slotsand to the platen enables it to dissipate sounds that would otherwiseemanate from the slots.

An additional unique feature of the present invention is its relativelysmall size which enables it to be readily removed from or placed on themachine with minimum difficulty. When it is not being used, it can bestored in a small volume.

The foregoing and other features and advantages of this invention willbe apparent from the following more particular description of thepreferred embodiments of the invention as illustrated in theaccompanying drawings.

In the drawings:

FIG. 1 is a perspective sketch of a typical typewriter having theacoustical cover of the present invention mounted thereon.

FIG. 2 is a perspective sketch of a partial end view of a typicaltypewriter having the acoustical cover of the present invention mountedthereon.

FIG. 3 is a perspective sketch of a partial front view of a typicaltypewriter which has been modified to re ceive the acoustical cover ofthe present invention.

FIG. 4 is a cross-section end elevational view of the acoustical coverof the present invention and a portion of a typewriter.

FIG. 5 is a schematic diagram of typical operator viewing positions withrespect to a typewriter having the acoustical cover of the presentinvention mounted thereon.

FIG. 6 is a schematic diagram of light rays and re flected light raysalong the typical operators line of sight to the acoustical cover of thepresent invention.

FIG. 7 is a partial front view of the foam strips of the acousticdevice.

FIG. 8 is a diagram of a basic low pass acoustic slot filter.

FIG. 9 is a frequency plot showing the sound level emitted from atypical typewriter compared with that of various configurations of theacoustical cover of the present invention.

DESCRIPTION Referring now to the drawings, and more particularly to FIG.1 thereof, a perspective sketch of a typical typewriter having theacoustical cover of the present invention mounted thereon is depicted.The typewriter 11 is a single element typewriter incorporating atypewriting mechanism similar to that described in US. Pats. 2,879,876and 2,919,002 assigned to the same assignee as the present application.The outer cover design of the the typewriter 11 enclose the platen 13and paper bail referenced copending application of Eliot F. Noyes andincludes an opening above the platen 13 to facilitate operator access tothe paper 15 and the print element 17. The acoustical cover 19 isadapted to fit over the platen opening and rests upon the outer coversof th typewriter 11 as will be described.

When in its fully operable position as depicted in FIG. 1, theacoustical cover 19 and the outer covers of the typewriter 11 enclosethe platen 13 and paper bail 21 as well as the print element 17 and thusforms a container therefore. In order to facilitate the feeding of paper15 around the platen 13 without necessitating the removal or opening ofthe acoustical cover 19, a paper bail release lever 23 is connected tothe paper bail 21 to ettect the conventional movem nt of the paper bailtoward and away from the platen 13. Additionally, operation of the feedroll release lever 25 effects the pivoting of a paper stacker (notshown) against the platen to facilitate the alignment of paper as it isinserted into the typewriter. The operation of this mechanism can beidentical to that described in the afore-referenced IBM Selectriccomposer instruction manual at page 15-1.

An additional modification which may be made to the typewriter machine11 to facilitate paper handling when the acoustical cover 19 is mountedthereon, is the inclusion of a paper guide 27 upon which the paper mayrest when it is being inserted into the machine. The paper guide 27 canbe mounted upon the fixedly secured acoustic cover support member 29.

When it is desirous to insert the paper 15 into the typewriter 11, theoperator moves the feed roll release lever 25 to effect thedisengagement of the feed rolls (not shown) from the platen 13, theengagement of the paper stackers (not shown) with the platen, and themovement of the paper bail 21 from the platen. Thereafter, the paper 15is inserted into a slot (not shown) created between the acoustical cover19 and the cover support member 29 until the paper rests upon the paperstackers. With the paper thus squarely aligned, the feed roll releaselever 25 is returned to its initial position thereby effecting theengagement of the feed rolls with the platen and the subsequentdisengagement of the paper stackers from the platen. Thereafter, theplaten 13 is rotated in the conventional manner effecting the feeding ofthe paper around the platen past the type element 17. Manipulation ofthe paper bail release lever 23 returns the paper bail 21 to the platenafter the paper is fed thereunder in order to insure contact of thepaper 15 with the platen 13. Continued rotation of the platen will causethe paper to be fed through a second slot (not shown) in the acousticalcover 19 as will be described hereinafter.

With the paper 15 thus inserted in the machine, the operator can readilyview the print line 31 through the transparent from wall 32 of theacoustical cover 19. Additionally, the operator can also view the linesimmediately preceding the print line through the front wall 32.

Referring now to FIG. 2 of the drawings, a perspective sketch of apartial end view of the typewriter 11 having the acoustical cover 19mounted thereon is depicted. When thus mounted on the typewriter, theacoustical cover 19 forms a paper passageway, slot 33 with the coversupport member 29 through which paper may be inserted into thetypewriter. A second paper passageway, slot 35 formed within theacoustical cover is provided for paper to emanate thereform. Theacoustical cover, when viewed from its end, is in the general shape ofthe letter M. The left-hand portion of the cover as viewed pivots in thedirection of arrow 37 about pivot studs 39 and 41 (FIG. 1) from aposition wherein the front wall 32 engages the lip surface 43 of thetypewriter cover to a position wherein the wall 45 of the acousticalcover rests against the wall 47 of the acoustical cover. In its thusopen or inoperative position, the operator may readily access the :printelement 17 or the paper 15 (FIG. 1) at the writing line in order to makecorrections, print element changes, etc. A partially open cover isdepicted in phantom.

A pair of mounting screws 49 and a similar pair (not shown) located atthe opposite end of the acoustical cover 19 fixedly secure an acousticdevice 50 to the end wall 51 and the opposite end wall (not shown) ofthe acoustical cover 19.

Referring now to FIG. 3 of the drawings, a perspective sketch of apartial front view of the typewriter 11 which has been modified toreceive the acoustical cover of the present invention is depicted. Thecover support member 29 is shaped to form a continuous surface with thesurface 52 of the cover of the typewriter 11. The surface 52 correspondsto the paper table which supports the the paper as it is inserted into astandard machine. With the machine modified to receive the acousticalcover, the surface 52 along with the cover support member 29 and thepaper guide 27 supports the paper 15 as it is inserted into the machineto be wound around the platen 13. The cover support member 29 has an endwall 53 and a second opposite end wall (not shown) located at each endthereof, each of the end walls containing a stud 55 on which theacoustical cover of the present invention rests. That is, the twoopposite end walls (i.e. wall 51 of FIG. 2) of the acoustical cover eachhave a slot (not shown) formed therein which engages the studs 55, theuppermost portion of the slot resting against the stud when the cover isin place. A small projection (not shown) located at the end edges of therear surface of the acoustical cover spaces the cover from the papertable and prevents it from rotating about the studs 55.

Referring now to FIG. 4 of the drawings, a partial cross-section endelevational view of the acoustical cover 19 and a portion of thetypewriter 11 is depicted. The front wall 32, the wall 45, the wall 47,and the end walls (end wall 59 of FIG. 2 and the end wall (not shown) atthe opposite end of the acoustical cover) :are formed of a transparentacrylic resin such as Lucite or Plexiglas. The front wall 32 as well asthe end walls are transparent. The wall 45 is frosted on each of itssurfaces to minimize glare and unwanted reflections of the printed pageoff of its surfaces. Additionally, the outside surface of the wall 47 isfrosted to further prevent glare and unwanted reflections. Although thewall 47 is frosted, it is sufiiciantly translucent for images appearingon the paper 15 immediately thereunder to be clearly discernable. Eachof the acrylic resin walls are approximately .100 inch thick in thepreferred embodiment.

An acoustic device 50 is located directly above the platen 13 andextends for approximately the entire length of the platen. The rearsurface 67 of the acoustic device forms the paper slot 33 with the papertable 52 of the typewriter and with a portion of the cover supportmember 29. The paper 15 is fed into the machine through this slot untilit engages the set of paper stackers (not shown). Thereafter, the rearfeed rolls (not shown) are caused to engage the paper so that it may befed around the platen 13 to the paper bail 21 as has been described.

As the paper 15 leaves the paper bail 21, it is directed toward theinclined surface 69 of the acoustic device 50. Thereafter, it is fedunder the radius 71 formed by the wall 47 of the acoustical cover 19 andthrough the slot 35 located between the wall 47 and the front surface 73of the acoustic device. The paper is thus fed through slot 35 withoutnecessitating operator intervention upon the rotation of the platen 13.

OPTICAL CHARACTERISTICS OF THE ACOUSTICAL COVER Referring now to FIG. 5of the drawings, a schematic diagram of typical operator viewingpositions with respect to the typewriter 11 having the acoustical cover19 of the present invention mounted thereon is depicted. Viewingposition 91 is representative of the eye location of a tall operatorwith respect to the typewriter 11 while viewing positions 92 and 93 arerespectively representative of the average typist and the short typistwith respect to the typewriter 11. Each of these viewing positionsremain unaltered when the acoustical cover 19 is placed on thetypewriter or removed therefrom. As can readily be seen from thediagram, each of the lines of sight from the respective viewingpositions to the print point 95 pass through the transparent front wall32 of the acoustic cover. It will further be noted that operatorvisibility of the paper 15 through the front wall 32 is at a maximum atthe viewing position 93 and the minimum at the viewing position 91 sinceall printing will appear on the paper 15 at or above the print point 95.Since the wall 45 ditfuses light rays, viewing of paper 15 through thesurface is precluded. Thus, at viewing positions 91 and 93, a portion ofthe previously printed text is not clearly discernable since the surface45 interferes with the operators view through the wall 32 of this text.It should be noted, however, that the operator at viewing position 91can see the print line as well as 3 or 4 lines there-above while theoperator at viewing position 93 can see the print line as well as 9 or10 lines there-above. Thereafter, a slight discontinuity in the view ofthe printed lines appears until the operator has a direct line of sightthrough the Wall 47 to the paper 15. The wall 45 is angled so as tocorrespond to the line of sight 97 between the average viewing position92 and the upper-most portion of the front wall 32 thereby effecting aminimum blockage of the average operators view by the wall 45.

It has been found experimentally that the viewing position 91 of atypical tall female operator is 15 /2 inches above the axis of theplaten 13 and 14 /2 inches horizontally away from the axis of the platen13. The viewing position 93 represents the viewing position of a shortfemale operator typically found to be 13 inches above the axis of theplaten and 17% inches horizontally away from the platen.

Referring now to FIG. 6 of the drawings. A schematic diagram of lightrays and reflected light rays along the typical operators line of sightto the acoustical cover of the present invention is depicted. The lightray 99 is representative of the typical operators line of sight to thetransparent wall 32 of the acoustical cover 19 and thence to the printpoint 95. Light which is reflected 01f of the front wall 32 back alongthe line of sight to the operator thereby representing glare or unwantedreflections must emanate from the top surface 100 of the typewriter 11as depicted by the light ray 101 since the angle of reflection along thelight ray 99 is equal to the angle of incidence along the light ray 101with respect to the front wall 32. By utilizing a dark top surface 100(e.g. an outer cover painted black) unwanted reflection is virtuallyeliminated.

The angle that the front Wall 32 makes with horizontal plane 102 may bevaried as shown by the broken lines 32' and 32". When the front wall 32is caused to slope outward as depicted by the front wall 32, light rays103 are caused to be reflected off of the surface of the front wall 32and transmitted back along the operators line of vision as depicted bythe light ray 99. The light ray 103 is depicted as a horizontal lightray. In most environments, such horizontal light rays emitting fromlight sources such as windows would be blocked by the operator prior tostriking the wall 32. However, if the wall 32' were sloped outward to agreater degree than depicted, unblocked light rays emanating fromwindows and from overhead light sources would be reflected off of thesurface of the front wall 32' back into the eyes of the operatorcreating undesirable glare.

Movement of the front wall 32 in the opposite direction as depicted bythe front wall 32" causes light rays 105 which are incident on thesurface of the wall 32" to be reflected back along the operators line ofvision. Since such light rays would have to emanate from within themachine, the chances of such light rays producing glare to the operatorare negligible. However, such inward movement of the front wall 32reduces the operators viewing window through which the print point andthe lines of print immediately above it are observed. This could in turnbe somewhat alleviated by increasing the height of the front wall 32with a resulting increase in size of the acoustical cover 19.

It has been found that for a preferred embodiment wherein the front wall32 has a height of 2 inches above the top surface of the typewriter 11,the front wall may be angled as depicted by front wall 32' byapproximately 20" with respect to an imaginary plane which isperpendicular to the horizontal surface 100 without incurring anappreciable amount of undesirable reflection or glare. The front wall asdepicted by front wall 32 may also be angled approximately with respectto the perpendicular plane without incurring an appreciable reduction ofthe operators viewing window. This assumes an environment wherein thelight source is either above the typewriter (e.g. ceiling lighting) orhorizontal (e.g. windows in sidewalls as found in a typical businessoflice. Thus, in a typical office environment, the front wall should besubstantially vertical.

As has been described, the wall 45 is angled to approximate the typicaloperators line of vision to the top of the wall 32 as depicted by lightray 97. In a preferred embodiment, this angle has been found to beapproximately with respect to the horizontal when the height of thefront wall 32 above the surface 100 of typewriter is approximately 2inches. This angle increases if the height of the front wall isincreased. The wall 47 forms a 102 angle with respect to the wall and isapproximately parallel to the paper 15 as it is emitted from theacoustical cover 19. In this manner, the paper is located directly underthe Wall 47 for ready visibility through the translucent but yetdiffused surface of the wall 47.

The wall 107 makes approximately a 35 angle with respect tohorizontal soas to direct any acoustical noise emitting from the paper slots formedtherein upward and toward the rear of the machine. The junction of thewalls 47 and 107 is approximately 1.4 inches above the uppermost surface100 of the typewriter in the preferred embodiment.

SOUND CHARACTERISTICS OF THE ACOUSTICAL COVER Referring once again to-FIG. 4 of the drawings, it has been described how the acoustical cover19 when placed on the typewriter 11 completes the enclosure of the areaaround the platen 13 of the typewriter. The only openings in the cover19 through which sound may emit are the slots 33 and 35 through whichthe paper 15 passes. In order to greatly reduce the noise passingthrough the slots, an acoustical device is located in close spacedrelationship to the platen and to the slots and absorbs the sounds thatwould otherwise emanate therethrough.

The acoustic device 50 can be a large open cavity or an acoustic filter.The preferred embodiment of the acoustic device 50* is an acousticfilter depicted in FIG. 4 of the drawings. In this embodiment, theacoustic device 50 is formed by two bent-sheet metal walls 111 and 113which, together with two end pieces (not shown), form a large cavity115. The end pieces (not shown) are of the same cross-sectional shape asthe cavity 115 and are fixedly secured to the end wall 51 (FIG. 2) andthe opposite end wall (not shown) of the acoustical cover 19 by mountingscrews 49 (FIG. 2) and a set mounting screws (not shown). In thepreferred embodiment, the acoustic device is 15.4 inches long,approximately the length of the platen 13 and the cross-sectional areaof the cavity 115 is approximately 0.8 inch by 1.9 inches orapproximately 1.5 square inches.

The cavity 115 is filled with four strips of inch thick open cellpolyurethane foam material 117, 118, 119, and 120. Any form of soundabsorbing material which has good high frequency absorptioncharacteristics such as fiberous glass or open cell sponge rubber couldalso be utilized in lieu of the polyurethane foam in order to absorb anddissipate sound. The strips of polyurethane foam material 117 and 120have a plurality of 0.44 inch diameter holes 122 located therein asdepicted in FIG. 7. The holes are located on horizontal centers .625inch apart and on vertical centers .56 inch apart. The percent of openarea of the foam is approximately 48% of the total surface area of foamstrip which extends almost the length of the acoustic device 50 orapproximately 14.4 inches in the preferred embodiment. The polyurethanefoam material members 118 and 119 are separated from one another by astrip 123 made of a polyethylene terephthalate such as Mylar which iscoated with pressure sensitive adhesive on either side. The strip 123runs the entire length of the acoustic device and thus creates twoacoustic cavities 125 and 126 within the cavity 115, the acoustic cavity125 being located adjacent the slot 35.

The two bent sheet metal members 111 and 113 each have a grill-likeopening member 128 and 129 located therein, each of the grill-likeopening members being contiguous to the slots 35 and 33 respectively,and each running for approximately the entire length of the slot orapproximately 14.4 inches in the preferred embodiment. The openings areformed by eleven horizontal rows of staggered .077 inch diameter holesplaced horizontally of an inch from one another and vertically of aninch from one another yielding an open area of approximately 45% of thetotal area of the grill-like opening members. Each of the grill-likeopening members are approximately 1.02 inches wide.

In order to determine the effective frequency range of an acoustic slotfilter, it is necessar to make various calculations which are based onthe dimensions of the filter and associated mass. Thus, in thediscussion which follows, reference will be made to an acousticalcompliance and to an acoustical mass there being an acousticalcompliance and an acoustical mass associted with each of the slots 33and 35. With reference to the slot 35, the acoustical complianceconsists of the cavity 126 and that portion of the slot 35 adjacent tothe grill-like opening member 128. The acoustical mass is the air masslocated in that portion of the slot located above the radius 71 of thewall 47 which is not adjacent to the grill-like opening member 128.Thus, the acoustical mass is located in a lower portion of the slot 35located above the radius 71 and below the grill-like opening member 128and in an upper portion of the slot 35 located above the grill-likeopening member and below the outer edge of the slot 35.

The most basic form of a low pass acoustical slot filter is shown inFIG. 8. The acoustic masses (M represents the mass of air in the openslot which is accelerated by a net force that acts to displace the gas(air) without compressing it. The acoustic compliance (C is associatedwith a volume of air that is compressed with a net force withoutacceleration. The cut-off frequency of the slot filter is inverselyproportional to the acoustic mass and acoustic compliance. The acousticmass of a narrow slot is given by the equation A 5 wt where l=length ofslot in which the sound wave is travelling,

w=width of the slot as viewed from the direction from which the soundwave is coming,

t=thickness of the slot p =density of air which is equal to 1.19 kg./m.

For the preferred embodiment of the acoustic device 50 of FIG. 4

l=0.25 in. t=0.078 in. w=14.4 in.

Hence, M =12.5 kg./m. The equivalent mass of the acoustic device isequal to 2M representing the mass on either side of the acousticcompliance which is equal to 25 kg./m. in the preferred embodiment.

The acoustic compliance of an enclosed volume of air V with an openingfor entrance of pressure variations is given by the following equation:

V=volume of air =density of air 0 =speed of sound=344.8 m./sec.

For the acoustic device depicted in FIG. 4,

V=(l4.4 in.) 0.4 in.) 1.9 in.)

Hence,

V CA -(1.263)(1O 9) newton The cut-off frequency of the filter is givenby the following equation:

The above computation shows that the acoustic device acts as a filterwhich effectively eliminates sounds emitting through the slots 33 and 35of FIG. 4 which have a frequency greater than 1790 c.p.s.

Referring once again to FIG. 4 of the drawings, it has been describedthat an alternate acoustic device 50 in the form of a large open chambercould be utilized instead of the acoustic filter depicted. Such a devicecould be shaped in the identical shape of the acoustic filter depictedwith the lower-most surface 131 of the sheet metal wall 113 removedtherefrom thereby creating an inverted U-shaped cavity. Additionally,the grill-like opening members 128 and 129 would be replaced by solidwalls and the inside surfaces of the thus formed inverted U-shapedmember can be lined with 4 inch thick sound absorbent material therebycreating a large sound absorbing surface directly over the platen areaand located immediately adjacent to the slots 35 and 33 through whichthe paper emanates. Such a device appreciably reduces emanated sounds offrequencies above 1,000 c.p.s.

Referring now to FIG. 9 of the drawings, a frequency plot showing thesound level emitted from a typical typewriter compared with a typewriterhaving the acoustical cover of the present invention incorporating theacoustic device 50 depicted in FIG. 4 and compared with a type writerhaving the acoustical cover of the present invention incorporating aninverted U-shaped acoustic device as heretofore described is depicted.The sound was measured by a microphone placed parallel to the paper pathand one foot above the top of the acoustical device. The ordinate of thefrequency plot represents the RMS sound pressure level as measuredwithin each octave pass band denoted on the upper abscissa scale. Thelower abscissa scale denotes the center frequencies of the octave passbands.

The curve 138 is that of a typical typewriter operating without anyacoustical cover over the platen area, the curve 139 is that of the sametypewriter operating with the acoustical cover of the present inventionhaving an inverted U-shaped acoustic device and the curve 140' is thatof the same typewriter operating with the acoustical cover of thepresent invention having an acoustic filter type acoustic device. As canbe seen, the acoustical cover incorporating either acoustic deviceeffects a 3 db sound reduction in the 710 to 1400 cycle per second passband and an 11 db reduction in the 1400 to 2800 cycle per second passband as compared with the typical typewriter.

As has been described, the cut-off frequency of the acoustic filter inthe preferred embodiment is 1790 cycles per second. Thus the curve 140is appreciably below the curve 139 in the frequency bands above 1790c.p.s. representing the sound diminuation effected by the filter.However, it should be noted that the acoustical cover of the presentinvention effects a substantial noise reduction in the frequency bandsabove 1790 c.p.s. when either acoustic device is incorporated therein.

While the above discussion has related to an acoustical cover for theplaten area of a typewriter or the like machine, it is understood bythose skilled in the art that noise which emits from other locations oftypical typewriters should also be minimized in order for there to be asubstantial reduction in the noise generated by such a machine. Forexample, the keyboard opening and slots such as the margin lever slotscould be sealed by utilizing respectively, a flexible membrane fittedover the keystems and a closed cell foam material fitted within the slotareas. Additionally, the interior walls of the machine could be linedwith sound absorbent material Where possible, and the machine could beshock mounted on its base in accord with conventional and well-knownpractices.

A further modification which could readily be made to the acousticalcover of the present invention would be to mount the end walls thereofon the typewriter housing. Thereafter, the remaining portion of thecover could be placed on the machine to form an integral container overthe platen area as has been described.

While the invention has been particularly shown and described withreference to the preferred embodiment thereof, it should be understoodby those skilled in the art that the foregoing and other changes in formand detail may be made therein without departing from the scope of theinvention.

What is claimed is:

1. An acoustical cover for an office machine of the type having ahousing for enclosing the machine operating elements and an opening foraccommodating paper access, said cover being configured to enclose saidopening and comprising:

a first substantially vertical transparent window wall;

a second wall connected to the upper portion of first wall and slopingdownward therefrom;

a third wall connected to the lower portion of said second wall andsloping upward therefrom;

a pair of oppositely faced end walls connected to the side edges of saidfirst, second, and third walls;

an acoustic device having one surface thereof substantially parallel tosaid third wall and forming an unobstructed paper feeding slot therewiththrough the enclosure formed by said cover, said acoustic devicedefining at least one acoustic cavity located in close proximity to thepaper feeding slot for reducing the noise that would otherwise emanatefrom the paper feeding slot.

2. The acoustic cover set forth in claim 1, wherein said first and saidsecond walls are pivotally connected with respect to said acousticdevice.

3. The acoustical cover set forth in claim 1 wherein said second andthird walls are constructed of a transluscent light diffusing material.

4. The acoustical cover set forth in claim 1 wherein said acousticdevice comprises an acoustic slot filter hav ing an opening between saidslot and said cavity whereby said cavity forms an acoustic compliancewith the acoustic air mass within said slot, said acoustic slot filteracoustically filtering audible sounds that would otherwise emanate fromthe slot.

5. The acoustical cover set forth in claim 1 wherein said acousticdevice comprises an inverted U-shaped member which forms said cavity,one wall of said member providing said substantially parallel surface,the open ing in said inverted U-shaped member being oriented directlyabove the machine operating elements and adjacent to the paper feedingslot created by said surface.

6. The acoustical cover set forth in claim 1 further compnsrng:

a fourth wall substantially perpendicular to the pair of end walls andforming a second unobstructed paper feeding slot with a second surfaceof said acoustic device.

7. The acoustical cover set forth in claim 6 wherein said acousticdevice comprises two acoustic slot filters each having an acousticcavity associated therewith and each being associated with one of saidslots, each of said slot filters having an opening between itscorresponding slot and cavity whereby each of said cavities forms an 1112 acoustic compliance with the acoustic air mass in its cor- 9. Theacoustical cover set forth in claim 8 wherein responding slot, each ofsaid acoustic slot filters acoussaid hood forms an integral portion ofsaid elongated tically filtering audible sounds that would otherwiseempassageway. anate from its associated slot. References Cited 8. Anacoustical cover for an office machine of the 5 UNITED STATES PATENTStype having a housing for enclosing the machines operating elements andopening for accommodating paper ac- 2511147 8/1950 cess, said acousticalcover comprising: FOREIGN PATENTS a hood configured to enclose saidopening and having 492 502 9/1938 Great Britain at least oneunobstructed paper slot located therein; 10 an acoustic slot filterhaving at least one elongated un- THER REFERENCES obstructed p p pahaving openings at Acustica (periodical) vol. 12, No. 3, 1962, pp. 139-PQ ends to facllltate the feeding of Paper from 152, The Performance ofAcoustic Hoods at Low Fre- Sald P p slot to the machlne Operatingelements, quencies, by R. S. Jackson, copy in Patent Office sciensaidfilter further comprising at least one acoustic 15 tifi 111 cavityforming an acoustic compliance with the acoustic air mass in saidpassageway for filtering ROBERT D, 111., Pnrnary Examiner audible soundsover a wide frequency range that US. Cl. X.R.

would otherwise emanate from the paper slot. 17842; 181-48

